1. Field of the Invention
The present invention relates to a brushless motor in which a circuit board is fixed to an attachment plate for use in attaching the brushless motor to a motor-carrying device.
2. Description of the Related Art
A brushless motor generates a rotational driving force as an electric current that is supplied to a specified one of a plurality of coils depending on the position of magnetic poles of a rotor magnet. The respective coils are supplied with the electric current through a coil wiring and the like formed on a circuit board.
In addition to the coil wiring, a Hall element for detecting the position of magnetic poles of the rotor magnet and a pattern of a frequency generator coil for detecting the rotational speed of a rotor are formed on the circuit board. The Hall element outputs a hall signal depending on the change in a magnetic flux density of the rotor magnet caused by rotational movement of the rotor. The frequency generator coil outputs a frequency generation signal depending on the change in a magnetic flux density of a frequency generation magnet caused by rotational movement of the rotor. The brushless motor makes use of a control unit that controls the amount of electric current supplied to the respective coils using the hall signal and the frequency generation signal.
Accordingly, if the magnetic flux density of the rotor magnet or the frequency generation magnet is changed due to vibration of the circuit board or other causes, it is impossible to accurately acquire the hall signal and the frequency generation signal. In an effort to prevent vibration of the circuit board, the circuit board is fixed to an attachment plate or the like which is to be attached to a housing or other devices. For example, Japanese Patent Application Publication No. 2006-158013A discloses a brushless motor in which a circuit board is fixed to a housing thereof.
In the brushless motor set forth in Japanese Patent Application Publication No. 2006-158013A, the circuit board having a coil wiring is fixed to the housing. More specifically, the hosing is provided with a support surface for supporting the circuit board and a plurality of protrusions. In a state that the lower surface of the circuit board is supported on the support surface, the protrusions are deformed to restrain the upper surface of the circuit board. By restraining the opposite surfaces of the circuit board in this manner, the circuit board is fixed to the housing against movement.
With the brushless motor disclosed in Japanese Patent Application Publication No. 2006-158013A, however, the fixing position of the circuit board is confined to the vicinity of a rotational axis of the motor, which means that the peripheral edge region of the circuit board is not in a firmly fixed state. For that reason, the region of the circuit board distant from the rotational axis is susceptible to vibration. As already mentioned above, it is impossible to accurately acquire a hall signal and a frequency generation signal if the circuit board undergoes vibration.
Aside from the brushless motor described in Japanese Patent Application Publication No. 2006-158013A, it would be thinkable that a circuit board is fixed to an attachment plate by means of screws. More specifically, it would be possible to suppress vibration of the circuit board as a whole by screw-fixing the circuit board to the attachment plate in plural points on the peripheral edge region of the circuit board. In case of screw-fixing the circuit board to the attachment plate, however, a problem occurs in that the number of parts and the number of production steps are increased.